This invention relates to an integrated circuit. In particular, this invention relates to an integrated circuit comprising a thermal conductivity based gas sensor.
Nowadays, integrated circuits may comprise a plethora of different sensors, such as ambient light (AL) sensors, temperature (T) sensors, gas sensors, relative humidity (RH) sensors, specific analyte detection sensors, and so on.
Integrated circuits of this kind have a wide range of applications. For example, they can be used in the field of supply chain management to track and monitor the freshness of food and beverages. They can also be used as environmental sensors, for example as part of a heating, ventilation and air conditioning (HVAC) system in an automobile or in a building (e.g. a Smart Building). Additional applications include those in agricultural (e.g. the sensing of environmental conditions in greenhouses) or in medical fields. Their provision in mobile communications devices such as mobile telephones, tablets or laptops can also enable a wide range of further applications that require measurements of local environmental factors.
The provision of sensors in integrated circuits of this kind allows devices to be produced that have a small form factor. For example, due to their small form factor, integrated circuits incorporating one or more sensors can be included in Radio Frequency Identification (RFID) tags, allowing for easy programming and readout.
Moreover, it allows large numbers of sensors to be manufactured cheaply, using established semiconductor processing techniques.
One kind of sensor that can be implemented in an integrated circuit is a thermal conductivity based gas sensor. These operate by passing a current through an electrically resistive sensor element, which causes the sensor element to heat up. The heat that is generated is dissipated by the surrounding environment including the substrate of the integrated circuit and also by any gas that is present in the vicinity of the sensor element. The resistivity of the sensor element is proportional to its temperature, and the temperature of the sensor element is in turn sensitive to the thermal conductivity of the gas (which governs the rate at which the gas can conduct heat away from the sensor element). Since the thermal conductivity of the gas is determined by its density and composition, it follows that electrical resistance measurements of the sensor element can therefore provide information about the gas. Since different gases have different thermal conductivities, the sensor can in principal be used to determine the presence and composition of a gas.
Thermal conductivity based gas sensors of this kind are known to be affected by cross sensitivity to relative humidity and ambient temperature. In order to correct for these factors, it is customary to provide a separate relative humidity sensor and temperature sensor. Optimally, these additional sensors would be provided in close proximity to the gas sensor. However, integration of such a diverse range of sensors on a single substrate is difficult. Accordingly, the additional sensors are typically provided on a separate substrate and/or in a separate package.
Another issue affecting thermal conductivity based gas sensors of this kind is the fact they operate at relatively high temperatures (which can be as high as 300° C.). Dissipation of heat into the substrate is problematic in the sense that it can cause unwanted heating of the other components in the integrated circuit, and also increases the amount of power needed to operate the sensor.